A vehicle lamp can generally switch between a low beam and a high beam. The low beam provides a predetermined illumination for a nearby area and has light distribution designed to not give glare to an oncoming vehicle or a preceding vehicle, so that the low beam is mainly used when travelling in urban areas. The high beam provides a bright illumination for a front wide and distant area and is mainly used when travelling at high speed on a road with few oncoming vehicles or preceding vehicles. Therefore, although the high beam gives better visibility to a driver than the low beam, the high beam would give glare to a driver of a preceding vehicle or a pedestrian at a front side of the vehicle.
In recent years, there has been proposed Adaptive Driving Beam (ADB) technique which controls a light distribution pattern of a high beam dynamically and adaptively based on conditions surrounding the vehicle. The ADB technique reduces glare to a vehicle or a pedestrian by detecting presence of a preceding vehicle, an oncoming vehicle or a pedestrian at a front side of the vehicle and reducing or turning off lights for an area corresponding to the detected vehicle or pedestrian.
FIG. 1 is a circuit block diagram of a vehicle lamp 100R studied by the inventors of the present application. The vehicle lamp 100R includes a lighting circuit 200R and a light source 300.
The light source 300 includes a plurality of (N; N≥2) light emitting elements 302_1 to 302_N. The lighting circuit 200R is configured to independently control turning-on/off of the light source 300 by a bypass method. The lighting circuit 200R includes a constant current circuit 202R, a bypass circuit 280, and a bypass controller 290.
The constant current circuit 202R generates a drive current (lamp current) ILAMP stabilized at a target value. The bypass circuit 280 includes a plurality of bypass switches SWB1 to SWBN. A bypass switch SWBi (1≤i≤N) is provided between both ends of a corresponding light emitting element 302_i. The bypass controller 290 controls on/off of the plurality of bypass switches SWB1 to SWBN individually so as to obtain a desired light distribution pattern. When an i-th bypass switch SWBi is turned off, the lamp current ILAMP flows into the light emitting element 302_i, and accordingly, the light emitting element 302_i becomes a lighting-on state. When the i-th bypass switch SWBi is turned on, since the lamp current ILAMP flows through the bypass switch SWBi and no current flows into the light emitting element 302_i, the light emitting element 302_i becomes a lighting-off state.
The constant current circuit 202R includes a switching converter 204, a sense resistor RS, a current detection circuit 206, and a converter controller 208.
The sense resistor RS is provided on a path of the lamp current ILAMP, and a voltage drop proportional to the lamp current ILAMP is generated between both ends of the sense resistor RS. The current detection circuit 206 generates a current detection signal VCS based on the voltage drop of the sense resistor RS.
The switching converter 204 is a buck converter or a boost converter. The converter controller 208 controls the switching converter 204 such that the detection signal VCS approaches a reference voltage VREF corresponding to the target value of the lamp current. For example, JP-A-2014-180099 discloses a lighting control device.
The inventors of the present application have recognized the following problems after investigating the vehicle lamp 100R in FIG. 1.
There may be a period during which all of the plurality of light emitting elements 302_1 to 302_N are turned off (complete lighting-off state) according to a light distribution pattern. It is possible to achieve a sufficiently long-time complete lighting-off state by stopping the constant current circuit 202R to set the lamp current ILAMP to zero.
However, in a situation where a short-time complete lighting-off state occurs repeatedly (for example, a situation where PWM control is performed on each bypass switch of the bypass circuit 280), the constant current circuit 202R cannot be stopped in the complete lighting-off state. This is because a delay occurs when the constant current circuit 202R is stabilized to an operating state from the stopped state and the lamp current ILAMP is not stabilized at the target current during the delay, so that the luminance of the light emitting elements become unstable. Therefore, it is necessary for the constant current circuit 202R to continue generating a constant lamp current ILAMP even in the complete lighting-off state.
In the complete lighting-off state, if the switching operation of the switching converter 204 is maintained, the same amount of lamp current IREF as that in a normal lighting-on state continues flowing into a switching transistor M1 and the bypass switches SWB. Therefore, the lighting circuit 200R consumes electric power although the light source is turned off, which causes heat generation of the transistors configuring the bypass switches SWB. Furthermore, a transistor which is capable of withstanding heat is large in size and high in cost.
The present invention has been made in view of the above circumstances, and an aspect of the present invention provides a lighting circuit which is configured such that switching operation of a switching converter can be maintained which can reduce power consumption in a complete lighting-off state.
According to an aspect of the present invention, there is provided a lighting circuit configured to drive a light source including a plurality of light emitting elements connected in series. The lighting circuit includes a plurality of bypass switches respectively connected in parallel with the light emitting elements; a switching converter; and a converter controller configured to stabilize a lamp current generated by the switching converter to a first target amount in a lighting-on state where at least one of the plurality of light emitting elements is turned on, and to stabilize the lamp current to a second target amount smaller than the first target amount in a complete lighting-off state where all of the plurality of light emitting elements are turned off.
According to this configuration, since the current flowing into the plurality of bypass switches can be reduced in the complete lighting-off state, the heat generation can be reduced. Also, since heat generation amount is reduced, inexpensive parts which are small in size can be selected.
The converter controller may include a first controller configured to generate a first control pulse by a control method with a relatively high precision in the lighting-on state, a second controller configured to generate a second control pulse by a control method with a relatively low precision in a complete lighting-off state, and a driver circuit configured to drive the switching converter according to the first control pulse and the second control pulse.
The lighting circuit may further include a determination circuit which is configured to compare a voltage across the light source with a threshold voltage and determine that the light source is at the complete lighting-off state if the voltage across the light source is lower than the threshold voltage.
The lighting circuit may further include a bypass controller configured to control the plurality of bypass switches; and a determination circuit configured to detect the complete lighting-off state according to a control signal from the bypass controller.
According to another aspect of the present invention, there is provided a vehicle lamp. The vehicle lamp includes the light source which includes the plurality of light emitting elements connected in series; and the above-described lighting circuit configured to turn on the light source.
The vehicle lamp may further include a scanning optical system configured to receive light emitted from the light source and scan the front of the vehicle.
Incidentally, any combination of the above configuration elements, and the configuration elements and expressions substituted in methods, apparatus, systems, or the like are also effective as aspects of the present invention.
According to the above configuration, heat generation can be reduced in the complete lighting-off state.